The developmental origin of skeletal muscle cell lineages has been extensively studied in the last 50 years. PAX3 + /PAX7 + myogenic progenitors are the foundation for skeletal muscle development and give rise to muscle satellite cells that support skeletal muscle postnatal growth, homeostasis and regeneration. Research into muscle connective tissue interstitial cells established their supportive role in muscle patterning during development and regeneration. With recent advances in mouse genetic lineage tracing, single cell transcriptomics and specific antibodies, the origin and the contribution of non-myogenic interstitial populations to the myogenic lineage have been reexamined. Growing evidence suggests that subpopulations of muscle interstitial cells actively contribute to myogenesis and fuse with myofibers during development, postnatal growth, and regeneration. In this review, we focus on the recent research advances that highlighted the direct contribution of interstitial cells, particularly connective tissue cells, to skeletal muscle growth and repair. We stress the importance of further investigating the mechanisms by which interstitial cells contribute to a myogenic fate in both physiological and pathological conditions.

Efficient tissue regeneration requires the precise coordination of inflammatory and regenerative programs, principally mediated by monocyte-derived macrophages. However, the transcriptional wiring and epigenomic processes behind complex macrophage subtype specification and transition between the different states are not known. Here we have identified the transcriptional repressor BACH1 as a critical, cell-intrinsic regulator of monocyte-derived macrophage specification during skeletal muscle regeneration. Using a myeloid-specific BACH1 knockout mouse model, we demonstrate that BACH1 deficiency disrupts the temporal coordination of monocyte-to-macrophage differentiation, leading to aberrant macrophage subsets with concurrent opposing pro- and anti-inflammatory features. Single-cell RNA-sequencing profiling reveals that BACH1 controls a core transcriptional network, including Nfkb1, Cebpb, and interferon signaling, governing inflammatory resolution and functional macrophage specialization. Mechanistically, BACH1 loss accelerates macrophage differentiation but also affects its core cellular identity, resulting in sustained, rather than declining inflammatory programs including upregulation of Il1b and thus, defective tissue remodeling. These immune alterations compromise the paracrine landscape during regenerative inflammation and impair muscle stem cell differentiation. Our findings establish BACH1 as a molecular tuner or controller that integrates early innate immune signaling with regenerative output, positioning it as a central node linking transcriptional control, immune fate decisions, and tissue repair.

Organs comprise diverse cell types originating from shared or distinct lineages. During embryogenesis, mesodermal Pax7+ progenitors give rise to skeletal muscle as well as non-muscle lineages like dermis and adipocytes. Here, we asked whether Pax7+ cells retain multipotency during early postnatal limb muscle growth. Lineage tracing in neonatal mice revealed unexpected early postnatal plasticity, yielding multiple non-myogenic lineages, including a previously unrecognized subpopulation of fibro-adipogenic progenitors, termed Pax7FAPs. Using mouse models, we demonstrated that Notch signaling primes neonatal Pax7+ cells toward a fibrogenic molecular identity, biasing their trajectory away from myogenesis. Long-term tracing confirmed that neonatally generated Pax7FAPs persist into adulthood. Furthermore, adult muscle injury triggered de novo generation of Pax7FAPs, which exhibited higher proliferative capacity than resident stromal cells. This postnatal Pax7+ multipotency reveals an additional cellular contribution to muscle development and regeneration.

Spinal muscular atrophy (SMA) is due to a deficit in SMN, a ubiquitously expressed protein encoded by the Survival of Motor Neuron 1 (SMN1) gene. Recently, SMN-targeted disease-modifying treatments have greatly improved the clinical outcomes of this neuromuscular disease. However, uncertainties remain regarding their long-term efficacy and non-neuronal tissue involvement in disease progression. Skeletal muscle tissue and the muscle stem cells (MuSC) that sustain its postnatal growth and regenerative capacity are affected by SMN deficit. While a direct contribution of muscle tissue in the disease progression has been demonstrated, the extent to which MuSC are involved in this process remains to be established. Using SMA type II patient muscle biopsies and several mutant mouse models, we performed an accurate study of SMN role in MuSC function during postnatal growth and adulthood. We found that SMA type II patient muscles display a reduced number of quiescent PAX7+ MuSC. In SMA mice, we showed that SMN is an important regulator of myogenic progenitor fate during early postnatal growth, and that SMN deficit compromises MuSC reservoir establishment. In Pax7 Cre-driven conditional knockout mouse models, we demonstrated that deletion of a single Smn allele is sufficient to induce quiescent MuSC apoptosis in adult muscle, showing that high levels of SMN are required for the maintenance of the quiescent MuSC reservoir. We further established that depletion of MuSC yielded neuromuscular junction remodelling followed by a non-cell autonomous loss of part of the alpha motor neurons (MN) in the long term. Overall, our findings demonstrate an interdependence between quiescent MuSC and the MN reservoirs, supporting that MuSC may be important therapeutic targets for the long-term treatment of SMA. Moreover, we provide important insights into the specific SMN requirements of MuSC, which could be valuable for to the development of next-generation combinatorial therapies.

Muscle stem cells (MuSCs) are essential for skeletal muscle repair. Following injury, MuSCs reside in low oxygen environments until muscle fibers and vascularization are restablished. The dynamics of oxygen levels during the regenerative process and its impact on muscle repair has been underappreciated. We confirm that muscle repair is initiated in a low oxygen environment followed by gradual reoxygenation. Strikingly, when muscle reoxygenation is limited by keeping mice under systemic hypoxia, muscle repair is impaired and leads to the formation of hypotrophic myofibers. Sustained hypoxia decreases the ability of MuSCs to differentiate and fuse independently of HIF-1α or HIF-2α. Prolonged hypoxia specifically affects the circadian clock by increasing Rev-erbα expression in MuSCs. Using pharmacological tools, we demonstrate that Rev-ERBα negatively regulates myogenesis by reducing late myogenic cell fusion under prolonged hypoxia. Our results underscore the critical role of progressive muscle reoxygenation after transient hypoxia in coordinating proper myogenesis through Rev-ERBα.

Metabolic reprograming has been linked to epithelial-to-mesenchymal transition (EMT) in cancer cells, but how it influences EMT in normal cells remains largely unknown. Here we explored how metabolism impacts delamination and migration of avian trunk neural crest cells, an important progenitor cell population of the vertebrate embryo. We report that delamination exhibits a quiescent metabolic phenotype whereas migration is characterized by OXPHOS-driven metabolism coupled to distinct expression of metabolic, EMT and developmental genes. While glucose and glutamine are required for delamination and migration, we uncover a specific role for glutamine and its catabolizing enzyme glutaminase in the unfolding of NCC delamination. Namely, glutamine is required for nuclear accumulation of glutaminase, which interacts and cooperates with Wnt signaling to regulate EMT gene expression and cell cycle during delamination. Our data indicate that similarly to cancer cells, embryonic cells engage metabolic enzymes for non-canonical signaling functions to connect metabolism with EMT.

Presynaptic terminals of neuromuscular junctions (NMJs) are sensitive to glutamate, which contributes to NMJ plasticity and synaptic neurotransmission. However, the effect of glutamate on neurotransmission and its pharmacologic modulation in muscle pathologies are understudied. In this study, the efficacy of pharmacologic blockade of glutamate dehydrogenase (GLUD)-1 was investigated in mdx mice, a model of Duchenne muscular dystrophy. The GLUD1 inhibitor R162 mitigated the malfunctioning of NMJs by enhancing glutamate release from muscle fibers and increasing its availability in the muscle interstitium. Glutamate binding to its N-methyl-d-aspartate receptor on the presynaptic bottom of NMJs resulted in the increased release of acetylcholine and functional recovery of the action potential and muscle contraction, ultimately improving muscle performance. Finally, the GLUD1 inhibitor did not affect the homeostatic control of NMJs and the behavior of either healthy or dystrophic mice. This study suggests a promising and feasible therapeutic approach, based on muscle glutamate exploitation, to treating Duchenne muscular dystrophy, an unmet need in the clinic.

Duchenne muscular dystrophy (DMD) mainly affects young boys with out-of-frame mutations in the DMD gene, leading to dystrophin deficiency. This loss disrupts the assembly of the sarcolemmal dystrophin-associated glycoprotein complex, resulting in membrane fragility and damage during muscle contraction-relaxation cycles. Consequently, patients experience progressive muscle weakness, loss of ambulation and cardiorespiratory failure. Gene therapy represents one of the most promising therapeutic approaches, requiring rigorous preclinical validation of candidate strategies. While several preclinical models of dystrophin deficiency mimic point mutations or exon deletions, no existing rat model accurately replicates DMD gene duplications, which account for approximately 10

Duchenne muscular dystrophy (DMD) is a devastating X-linked neuromuscular disorder characterized by the absence of a functional dystrophin, leading to progressive muscle loss responsible for cardiorespiratory failure and premature death. While mouse, dog, and pig models have long supported DMD preclinical research, each has limitations in terms of phenotype severity, translational relevance, cost, or ethical acceptance. The emergence of genetically engineered DMD rat models marks a major advancement, offering an intermediate platform that combines practical handling, robust disease features, and disease trajectory accuracy with human patients. Rat models exhibit early, progressive and severe skeletal and cardiac pathology, including impaired muscle regeneration due to satellite cell senescence, all of which closely mirrors patient pathology. In vivo single-nucleus transcriptomics has further highlighted the complexity of fibrotic, inflammatory, and stem cell dysfunction across affected tissues. Importantly, DMD rat models have proven valuable for preclinical therapeutic studies, including gene and exon-skipping therapies, small compounds or cell-based interventions, and senescence-targeting strategies. They have also supported functional, histological, and molecular endpoints aligned with clinical practice. Importantly, DMD rat lines are not phenotypically uniform. Variations in mutation type, involvement of specific dystrophin isoforms, spontaneous exon skipping, and genetic background lead to differences in disease onset, severity, organ involvement, and survival. These distinctions influence the suitability of each model for precision therapeutic strategies. DMD preclinical rat models therefore provide a powerful complementary tool that fits into a continuum of modeling to advance understanding of pathogenic mechanisms, biomarker discovery, and translational research. Their progressive adoption will be accelerating the development of more effective and clinically relevant therapies for patients affected by dystrophin deficiency.

H3.3 histone chaperone DAXX regulates heterochromatin silencing; however, its function in transcription regulation remains understudied. Here, we show that Daxx knockout (KO) myoblasts have impaired differentiation and fusion. Transcriptomic analysis revealed a loss in myogenic gene expression and broad transcription dysregulation in Daxx KO myoblasts. Chromatin immunoprecipitation followed by sequencing demonstrated a marked reduction in H3.3 deposition at myogenic loci in Daxx KO myoblasts, which was further linked to decreased H3K27ac. Intriguingly, the double KO of Daxx and Hira resulted in distinct transcriptomic alterations than those of single KOs, demonstrating that DAXX and HIRA have both overlapping and unique roles in H3.3 incorporation. Our findings establish DAXX as a critical regulator of myogenic gene expression and muscle cell identity through a distinct mechanism from that of HIRA and highlighted an unanticipated plasticity in the deposition loci for DAXX and HIRA in myoblasts.

CACNA1S gene variants are associated with congenital myopathies (CMyo) with triad dysfunction (triadopathies), malignant hyperthermia susceptibility, hypokalemic periodic paralysis and thyrotoxic periodic paralysis. Here, we generated three iPSC lines derived from patients with CMyo linked to both autosomal dominant and recessive CACNA1S variants (CACNA1S-CMyo). The three lines displayed typical iPSC morphology, uniform expression of markers of the undifferentiated state, trilineage differentiation potential and normal karyotypes. As CACNA1S-CMyo are ultra-rare disorders, these lines enable a better in vitro characterization of CACNA1S pathophysiology and can be used to test different treatment approaches.

Muscle stem cells rely on their niche for maintenance, yet how β-adrenergic innervation regulates these cells remains elusive. Here, we show that sympathetic fibers in skeletal muscle innervate the vascular stem cell niche, specifically targeting β-adrenergic receptors on perivascular cells. We observe that sympathetic denervation leads to vascular remodeling and, concomitantly, reduces the muscle stem cell pool, resulting in tissue repair defects. Mechanistically, we demonstrate that sympathetic denervation reduces perivascular-derived angiopoietin-1, a crucial factor in maintaining the quiescent state of post-natal muscle stem cells. Using pharmacologic and genetic tools, we identify that sympathetic signaling drives angiopoietin-1 production from murine perivascular cells through the stimulation of their β-adrenergic receptors, thereby preserving the quiescent stem cell pool. Collectively, our data identify the molecular and cellular axis coupling skeletal muscle tissue homeostasis and regeneration to sympathetic innervation and β-adrenergic signaling, which are thus key signaling pathways that contribute to satellite cell quiescence.

Transposons and their derivatives make up a major proportion of the human genome, but they are not just relics of ancient genomes. They can still be expressed, potentially affecting the transcription of adjacent genes, and can sometimes even contribute to their coding sequence. Active transposons can integrate into new sites in the genome, potentially modifying the expression of nearby loci and leading to genetic disorders. In this review, we highlight work exploring the expression of transposons in skeletal muscles and transcriptional regulation by the KRAB-ZFP/KAP1/SETDB1 complex. We next focus on specific cases of transposon insertion causing phenotypic variation and distinct muscular dystrophies, as well as the implication of transposon expression in immune myopathies. Finally, we discuss the dysregulation of transposons in facioscapulohumeral dystrophy and aging.

Understanding how transcription factors regulate organized cellular diversity in developing tissues remains a major challenge due to their pleiotropic functions. We addressed this by monitoring and genetically modulating the activity of PAX3 and PAX7 during the specification of neural progenitor pools in the embryonic spinal cord. Using mouse models, we show that the balance between the transcriptional activating and repressing functions of these factors is modulated along the dorsoventral axis and is instructive to the patterning of spinal progenitor pools. By combining loss-of-function experiments with functional genomics in spinal organoids, we demonstrate that PAX-mediated repression and activation rely on distinct cis-regulatory genomic modules. This enables both the coexistence of their dual activity in dorsal cell progenitors and the specific control of two major differentiation programs. PAX promote H3K27me3 deposition at silencers to repress ventral identities, while at enhancers, they act as pioneer factors, opening and activating cis-regulatory modules to specify dorsal-most identities. Finally, we show that this pioneer activity is restricted to cells exposed to BMP morphogens, ensuring spatial specificity. These findings reveal how PAX proteins, modulated by morphogen gradients, orchestrate neuronal diversity in the spinal cord, providing a robust framework for neural subtype specification.

Duchenne Muscular Dystrophy (DMD) is a progressive neuromuscular disorder characterized by impaired muscle repair. Forskolin (FSK), an adenylyl cyclase activator, has shown potential in enhancing muscle regeneration and limiting muscle stem cell senescence. This study aimed to evaluate the effects of FSK on muscle repair, fibrosis, inflammation, and long-term muscle function in DMD using a preclinical rat model. BaCl2-induced muscle injury was performed on 6-month-old DMD (R-DMDdel52) and wild-type (WT) rats. FSK was supplied via short-term and long-term administration. Muscle tissues were harvested 14 days post-injury for histological analysis, including hematoxylin and eosin and Sirius red staining. Immunofluorescence was used to assess fibroadipogenic progenitors (FAPs), regeneration, muscle stem cells, and macrophage phenotypes. Moreover, we performed a study by chronically administering FSK to DMD rats from 1 to 7 months of age, either intraperitoneally (IP) or subcutaneously (SC). Functional assessments included grip strength test, in vivo muscle force measurements, plethysmography and electrocardiograms. Post-sacrifice, Tibialis anterior, diaphragm and heart tissues were histologically analyzed, to evaluate muscle architecture, fibrosis, and histopathological indices. FSK treatment significantly improved muscle histology and reduced fibrosis in both uninjured and injured DMD muscles by decreasing the number of FAPs. Long-term FSK treatment in the acute injury model enhanced muscle regeneration, increased MuSC proliferation, and reduced senescence. FSK also modulated inflammation by reducing pro-inflammatory macrophages and promoting a shift to a restorative phenotype. However, despite these histological improvements, FSK treatment from 1 to 7 months resulted in limited functional benefits and worsened ventricular histology in the heart. FSK shows promising results in improving muscle regeneration and reducing fibrosis in DMD, but concerns remain regarding its limited chronic functional benefits and potential adverse effects on cardiac tissue. Our results highlight the need for optimized adenylyl cyclase activators for therapeutic use in DMD patients.

Introduction Although human and animal exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) has been shown to be associated with cardiovascular diseases and death, the impact of long-term TCDD exposure on the heart remains poorly understood, especially in vulnerable populations. In male healthy mice, we have shown that TCDD exposure led to an early and progressive onset of endothelial damage and left ventricular (LV) diastolic dysfunction while preserving systolic function. However, it remains unclear whether chronic low-dose TCDD exposure accelerates the development and progression of cardiomyopathy in obese subjects. Additionally, there are few published studies investigating whether TCDD exposure leads to sex-specific metabolic cardiovascular outcomes in obese mice. Objective Our goal was to investigate the cardiovascular toxicity of prolonged exposure to TCDD in the context of obesity and to uncover potential sexual dimorphism. Method Six-month-old male and female C57BL6N mice were fed with normal diet or high fat diet (60 kcal% fat) and simultaneously exposed to vehicle or low-dose TCDD (4μg/kg, once per week, oral gavage) for 30 weeks. Results During the follow up, TCDD exposure had no impact on weight gain in obese mice, whatever the gender. After 30 weeks, HFD increased fat mass and glucose intolerance in both gender. TCDD exposure did not affect body fat or glucose tolerance in lean and obese mice of either sex. Echocardiography analysis showed that HFD induced LV systolic and diastolic dysfunctions only in male mice. Chronic TCDD exposure exacerbated LV diastolic dysfunction in male obese mice, while female mice were preserved. The worsening of diastolic dysfunction in obese male mice exposed to TCDD is associated with an increase in heart weight and cardiomyocyte size, without an increase in cardiac fibrosis. In both gender, AHR was expressed in obese heart, especially in cardiac endothelial cells that responded strongly to TCDD exposure by increasing CYP1A1 expression, whatever the diet. Using in vivo flow-mediated dilatation analysis, we showed that HFD did not affect endothelium-dependent arterial relaxation in both gender, whether chronic TCDD exposure led to strong endothelial dysfunction only in male obese mice, while female mice were protected. Conclusion In obese mice, prolonged low-dose TCDD exposure aggravated endothelial damage and LV diastolic dysfunction only in males.

Presynaptic terminals of neuromuscular junctions (NMJs) are sensitive to glutamate, which contributes to NMJ plasticity and synaptic neurotransmission. However, the effect of glutamate on neurotransmission and its pharmacologic modulation in muscle pathologies are understudied. In this study, the efficacy of pharmacologic blockade of glutamate dehydrogenase (GLUD)-1 was investigated in mdx mice, a model of Duchenne muscular dystrophy. The GLUD1 inhibitor R162 mitigated the malfunctioning of NMJs by enhancing glutamate release from muscle fibers and increasing its availability in the muscle interstitium. Glutamate binding to its N-methyl-d-aspartate receptor on the presynaptic bottom of NMJs resulted in the increased release of acetylcholine and functional recovery of the action potential and muscle contraction, ultimately improving muscle performance. Finally, the GLUD1 inhibitor did not affect the homeostatic control of NMJs and the behavior of either healthy or dystrophic mice. This study suggests a promising and feasible therapeutic approach, based on muscle glutamate exploitation, to treating Duchenne muscular dystrophy, an unmet need in the clinic.